A. Field of the Invention
The present invention relates to surface mount capacitors and, in particular, to surface mount capacitors having a capacitive element substantially encapsulated in a body or case.
B. Problems in the Art
Demand has steadily increased for surface mount capacitors. They are useful for numerous and widely-varying applications and functions. For example, they are useful for maintaining signal integrity and high speed delivery of charge in electrical and electronic components or devices. They are also particularly useful in switching functions. They are useful for bulk decoupling capabilities to smooth transient requirements seen by a power source.
The types and configurations presently available are numerous. Most have some type of capacitive element inside an enclosure or case. External conductive connections or terminations are electrically connected to the internal capacitive element. The capacitor assembly can be placed upon a circuit board and connected to the circuit through the terminations.
Different capacitive element configurations produce different capacitive performance. The nature of the capacitive elements can determine their size. For example, some need to handle high voltage and, to do so, must use relatively large capacitive elements. This results in a relatively large case size.
However, many times the size of electrical components is important in circuit design. This brings what is called “volumetric efficiency” into play. Volumetric efficiency is known in the art to refer to capacitance per unit volume. Two aspects of volumetric efficiency relative to the present invention are as follows.
First, there is volumetric efficiency of the capacitive element itself. Some materials have a higher capacitance performance than others for the same size or volume. A good example is tantalum. It is well-known that a solid tantalum capacitive element exhibits more capacitive performance than aluminum for the same volume.
Second, there is volumetric efficiency of the entire capacitor; namely the capacitive element(s), case, and terminations. The case defines a certain volume. If the volume of the capacitive element inside the case is small relative to the total volume of the case, the volumetric efficiency of the entire capacitor is normally lower than if the volume of the capacitive element is large relative to case size.
If room on the circuit board for the capacitor is not a concern, volumetric efficiency may not be a concern. However, as can be appreciated, volumetric efficiency becomes increasingly important as space for the capacitor becomes more limited. As increasing miniaturization occurs for a wide variety of electronic and electrical devices, demand increases for increasingly smaller surface mount capacitors.
Capacitors can represent the highest part count in many circuits. Therefore, a reduction in case size (and thus volume) of capacitors, while maintaining (or even increasing) capacitive performance, is an important present need in the art. Circuit designers need to be able to specify a certain case size for capacitors to allow them to fit on a circuit board with the other components needed for the electrical or electronic device.
However, it is difficult to simultaneously meet increasing capacitive performance needs and at the same time have a very small package or case size. Minimizing size while maintaining or improving capacitor performance is a challenging task. Additionally, independent of case size, there is always a need to improve the performance of, and volumetric efficiency of, capacitive elements and capacitor assemblies.
One way to improve volumetric efficiency is to use a high performing material, for example tantalum (Ta), Niobium (Nb), or Niobium Oxide (NbO), for the anode material. Solid core or pellet surface mount capacitors of this general type are well known in the art. Examples can be seen at U.S. Pat. Nos. 6,380,577 and 6,238,444, incorporated by reference herein. In those patents, the solid interior core (sometimes called an anode body, slug or pellet) is primarily Ta. The tantalum anode body is usually sintered. A wire is commonly formed in the anode body in one of two ways; (a) “embedded”, meaning the wire (also can be Tantalum) is covered with Tantalum powder during a pressing process or (b) “welded” meaning after the pellet is pressed and sintered, the wire is welded to the Ta slug. The other end extends outside the slug. The capacitor dielectric material is made by anodic oxidation of the anode material to form an oxide layer over the surface of the anode body (e.g. Ta→Ta2O5). If the anode body is Nb the oxidation is Nb→Nb2O5; if NbO, the oxidation is NbO→Nb2O5. A capacitor cathode is commonly formed by coating the dielectric layer with a solid electrolyte layer (e.g. of MnO2) and a conductive polymer, and later covered with graphite and silver for better conductivity and improved mechanical strength. Anode and cathode terminations can be connected to the free end of the Ta wire and the outer electrolyte surface coating of the Ta pellet, respectively, and all these components can then be encapsulated within a case (e.g. by molding plastic around the components), leaving only outer surface(s) of the anode and cathode terminations exposed on the exterior of the case for, e.g., surface mounting.
U.S. Pat. Nos. 6,380,577 and 6,238,444, describe surface mount tantalum capacitors of this general type. However, the terminations extend around the edges of the case ends in a U-shape. Therefore, they are known as “wrap around” terminations. As can be seen at FIG. 6 of U.S. Pat. Nos. 6,380,577 and 6,238,444, these “wrap around” portions (reference number 36) provide an anode/cathode termination pair in two planes or sides of the device. While this allows the device to be surface-mounted on one of two sides (they can be referred to as “two-sided terminations”), as compared to “single-sided” terminations, which can be surface mounted on one side only, it presents a problem. These “wrap around” or “two-sided” terminations can result in shorting between opposite ends of the device when in place on a circuit board. An example of this shorting problem exists in many radio frequency (RF) applications where metal shielding is placed over at least portions of the circuit boards. Portions of the conductive terminations extend up to and into the top plane of the capacitor case.
Therefore, there is a demand for capacitors with “single-sided terminations”, meaning an anode and cathode termination pair for surface mounting exist on only one side or plane of the device. One configuration for such capacitors is illustrated in the drawing of FIG. 13A, a solid slug (e.g. Ta) capacitor. This cross-sectional view shows a conventional tantalum slug or pellet 1 with an outward extending embedded tantalum wire 9 encapsulated in case 6 of plastic material. Anode termination 3 is positioned on what will be called the bottom surface of encapsulating material or case 6 directly underneath the free end of wire 9. A conductive adhesive 4 and an internal conductive path 15 electrically communicate the free end of wire 9 with anode termination 3 through the encapsulating material 6. A cathode termination 2 (also on the bottom side of the encapsulating material or case 6 but directly underneath the end of pellet 1 opposite wire 9) is electrically connected to the exterior of pellet 1 through another pad of conductive adhesive 4. Thus, in comparison to the wrap around terminations of the capacitors of U.S. Pat. Nos. 6,380,577 and 6,238,444, the capacitor of FIG. 13A has single-sided terminations. The anode and cathode terminations are in the same general plane on one side, the bottom side as shown in FIG. 13A, of the capacitor device. A similar prior art embodiment of such a single-sided termination capacitor is illustrated in FIG. 13B.
While the prior art capacitors of FIGS. 13A and B do not present the earlier discussed problem associated with the “wrap around” terminations of U.S. Pat. Nos. 6,380,577 and 6,238,444, they do present a volumetric efficiency issue. As shown in the cross section views of in FIGS. 13A and B, the encapsulating material of case 6 encases not only pellet 1 but all of the outward extending portion of wire 9. In particular, there is a substantial volume of case 6 between the distal end of wire 9 and the outer surface of case 6. Sufficient space must be allowed for the interior electrical connection or path 15 between wire 9 and anode termination 3. In essence, a rather substantial volume of encapsulating material in case 6 is used up to completely encase both the free end of wire 9 and the connection 15 between wire 9 and anode termination 3. This limits the size of Ta pellet that can be placed in case 6. A substantial volume of the entire capacitor case must be dedicated to the electrical connection of wire 9 to anode termination 3, as opposed to being filled more completely with pellet 1.
Therefore, a need in the art exists for a surface mount capacitor with improved volumetric efficiency.
Furthermore, it is difficult to optimize volumetric efficiency (capacitance per unit volume of the device) when manufacturing such capacitors, especially when the devices are of the smaller case sizes. It is difficult to control the thickness and uniformity of thickness of the encapsulating material around the capacitive element (e.g. pellet 1), both when molding the material around the pellet and when producing the final device. This is either ignored or tends to result in over-compensation in designs and manufacturing steps that result in thicker case walls which, in turn, limits space for the capacitive element. Many present state-of-the-art capacitors therefore have relatively thick case walls. Volumetric efficiency suffers.
As can be appreciated, these volumetric efficiency issues apply to other single-side termination surface mount capacitors as well. Any increase in volumetric efficiency can potentially be beneficial regardless of size or type of capacitor. A real need in the art has therefore been identified for an improved single-sided termination surface mount capacitor.